The field of vacuum microelectronics (VME) is directed to the miniaturization of vacuum tubes and other structures. In such devices, charged particles, e.g., electrons, flow within a vacuum environment between a cathode and an anode. In VME environments, electrons travel without scattering and are easily manipulated. (The rest mass of an electron is only 9.11×10−32 kg.) Thus, those devices can operate at high frequencies, and little if any heat is generated by the flow of electrons. Early vacuum tubes were developed to generate devices operating at 13 MHz. The speed of many vacuum tubes, has been traditionally limited by two factors. The first is capacitive damping of the applied biases on the plates and feed throughs into the glass envelope, or in the case of MEMS based devices, the leads into and out of the device. This issue was solved for large conventional tubes in part by utilizing resonant cavities to self-bias the tube at the resonant frequency of the cavity. The space-charge of the electron beam induces an electric field, and in turn, a current which travels around the cavity. The velocities of the electrons in the beam are influenced by these space-charge induced oscillating fields, making a tendency for the electrons to bunch up. This bunching in beam current intensifies to a maximum at some later time, and the resulting space-charge wave is coupled out to provide power at the resonant frequency of a cavity. The use of these resonant structures to avoid coupling high frequency signals through capacitively leaky feadthroughs increases the available oscillation frequencies to a regime where the second limiting factor becomes important. In large-scale tubes, the maximum frequency is limited by the resistance of the material from which the cavity is made. Since the resistivity of the materials being used generally increases with frequency through the gigahertz range, the frequency of operation is limited by resistive losses as a result of the currents flowing in the resonant structures. In addition, as the frequency increases, the size of the cavity becomes more difficult to manufacture by conventional means.
Known commercially used light generation techniques are very inefficient. Incandescent lamps (e.g., with tungsten filaments) waste greater than 90% of their energy in the generation of heat and infra-red electromagnetic radiation. Quartz lamps are twice as efficient as tungsten filament lamps (which are 40× better light emitters than candles).
In addition to incandescent lights, there are other sources of light. Fluorescent lights contain low pressure mercury vapor in a phosphor-coated glass tube which has electrodes at each end. When a current is applied across the electrodes, the electrons collide with and ionize the mercury vapor. The ionized mercury vapor emits some light in both visible and ultraviolet ranges. The visible light is emitted directly, while the ultraviolet light is absorbed by a phosphor and re-emitted as visible light. Because a greater fraction of the energy is consumed by light production rather than heating, fluorescent lights are more efficient than incandescent lights. The inside of the glass is coated with a phosphor, a material that is fluorescent. Phosphors absorb high-energy photons, and emit the light as lower energy photons; in this case, they absorb light in the ultraviolet range, and re-emit it as visible light. This can be explained by the fact that some of the electrons of the phosphor do not immediately drop all the way to their ground state, but relax to an intermediate state before dropping to ground state.
The second type of light is a sodium or neon-type light. These are filled with a particular gas at low pressures. Electric current passed through the gas causes electrons of the individual gaseous molecules to jump to higher energy states. They then decay to their normal state, emitting light of a characteristic wavelength: sodium lights are yellow, neon is red, and so on. Some elements, like sodium, give a single, very intense line when the light is passed through a prism to separate the colors (sodium lamps are 6-8× more efficient than incandescent lamps, because nearly all of their light is a single frequency rather than the mixture found in incandescent lamps). Other elements, such as helium or neon, give a series of lines when the light is passed through a prism, some of which are in the ultraviolet region; the characteristic color of these gases is determined by the combination of the relative intensities of the various bands.
LED (light emitting diode) technology has made recent gains beyond basic display applications to achieve use in low-end illumination applications (flashlights, automobile tail lights, etc.), but with great improvement in reliability over prior filament-based lamps. Economies of scale have continued to make the technology increasingly affordable. The basic mechanism of light emission in LEDs is the combination of electrons and holes to generate a photon. Because electrons must move through the semiconductor material, energy is lost as electrons collide with the bulk of the material.
Previously, light production in semiconductors suffered from several problems, including: (1) the inability of standard silicon-based processes to produce light directly, and (2) functional light emitting devices in production often use exotic and non-silicon materials (e.g., Group II-V materials) to produce light. These materials are generally not compatible with the production of highly integrated devices—such as microprocessors—due to cost and yield issues that cannot be cost effectively eliminated. Thus, the vast majority of chips that are produced today use standard silicon technology and suffer from the lack of light emitting devices.